1. Field of Invention
The present invention relates to a method of forming a laser induced grating pattern. More particularly, the present invention relates to the production of a laser induced grating pattern by providing a pulse laser, which is incident to an interference strain/displacement gage (ISDG).
2. Description of Related Art
With rapid progress in semiconductor production technology, integrated circuits continue to shrink and functional performance continues to improve. However, circuit miniaturization and performance improvement demands an increase in the level of integration. Because most integrated circuits are increasingly complicated, a multi-layer design is frequently required. In the fabrication of integrated circuits, processes including material deposition, photolithographic exposure, pattern etching and chemical-mechanical polishing are routinely deployed. Based on reliability consideration, thickness of thin films after a material deposition or a chemical-mechanical polishing operation is often measured. In general, a non-destructive measurement is preferred over other means of measuring the thickness of the film to ensure film integrity. The most common non-destructive method of measuring the thickness of a film includes providing two beams of laser which are incident at an angle to the film simultaneously.
One major aspect of film thickness measurement is the production of a laser induced grating pattern. In general, the profile, the degree of resolution between the bright and the dark pattern and the contrast (difference) in brightness between the bright and the dark pattern are principle factors that affect thickness measurement directly. Therefore, a variety of techniques for producing a laser induced grating pattern have been developed. The most common method for measuring the thickness of a film is still the application of two incident laser beams at an angle into one particular location on the film.
FIG. 1 is a perspective view showing two incident laser beams targeting a single spot on the surface of a film to produce a laser induced grating pattern together and associated film thickness measurement. As shown in FIG. 1, two pulsed laser beams 100a, 100b having an identical frequency (wavelength) aims at the same location on the thin film 102 at an angle of incident θ. Through the interference of the pulsed laser beams 100a, 100b, a grating pattern 104 (a laser induced grating pattern) is formed on the film 102. The grating pattern 104 generates a surface acoustic wave (SAW) 106 on the film 102 having a vibrating frequency in the megahertz (MHz) to the gigahertz (GHz) range. In fact, the surface acoustic wave 106 is a standing wave propagating parallel to the surface of the film 102.
The incident angle θ of the pulsed laser beams 100a, 100b largely determines the spacing between the bright and dark pattern as well as the wavelength of the surface acoustic wave 106 generated on the surface of the film 102. To facilitate thickness measurement of the film 102, the spacing of the bright and dark pattern and the wavelength of the surface acoustic wave 106 of the grating pattern 104 can be adjusted by changing the incident angle of the pulsed laser beams 100a, 100b. 
After producing the surface acoustic wave 106 on the surface of the film 102, a probing laser beam 108a is provided on the film 102 to produce a diffracted laser beam 108b. Through the laser beam 108b, the frequency and the velocity of the surface acoustic wave 106 can be deduced. Because the frequency and propagating velocity of the surface acoustic wave 106 is affected by the thickness of the film 102, thickness of the film 102 can be deduced from the information gather from the laser beam 108b about the surface acoustic wave 106.
However, providing two pulsed laser beams which are incident on the surface of a film to generate a grating pattern typically entails the following problems:                1. The grating pattern has an elliptical profile with low resolution and contrast between bright and dark pattern. Hence, the conventional technique can hardly produce a narrow surface acoustic wave. Furthermore, the signal/noise (S/N) ratio is rather low and hence leads to a low resolution for the film measurement.        2. Adjusting the angle of incident of the two pulsed laser beams accurately and simultaneously is a difficult feat. If there is minor nonconformity between the two incident angles of the laser beams, the spacing between the bright and dark lines in the grating pattern or the wavelength of the surface acoustic wave on the film is difficult to decide. In some cases, not even the grating pattern will form leading to the impossibility of measuring the film thickness.        3. The two-laser-beam method can be applied to measure a metallic film with a thickness of more than several thousand angstrom (Å). Hence, the method is unsuitable for measuring a film any thinner.        4. Setting up the two-beam system in the production equipments along a production line to measure film thickness is rather difficult because the beams and the surface film must be positioned very accurately.        